Following the development of the light/laser technology, transparent resin materials have been used for optical members such as optical lenses, prisms, and light guides for which transparent materials such as glass have been used. Resin optical materials are lighter than glass. Further, the resin optical materials have an advantage in that it is possible to easily mass-produce optical members having non-curved surfaces or complicated fine shapes that have been difficult with glass. For this reason, the optical devices using the resin optical materials have been remarkably reduced in weight and size than before.
For example, as optical lenses for a telephoto lens mounted in a high-performance camera, a plurality of curved glasses have conventionally been used in layers. Therefore, the telephoto lens has increased in size and weight and has been difficult to handle. However, by the use of non-curved resin lenses, it has become possible to largely reduce the number of lenses to thereby reduce the weight and size of the telephoto lens so that everybody can easily handle it.
Complicatedly shaped transparent resin optical sheets or plates are used also in flat panel liquid crystal displays which have recently increased in demand. Without these transparent resin optical members, reduction in thickness and weight of the flat panel displays is impossible. Particularly, large-size flat panel liquid crystal displays each having a screen diagonal of 28 inches or more which have rapidly increased in demand recently have a feature that they are much thinner and lighter than CRTs currently predominant in the displays. Such a large-size flat panel liquid crystal display is an epochal display that is easy to carry about, can be hung on the wall, and is capable of realizing space saving in the room. The realization of the reduction in thickness and size owes to the existence of the transparent resin optical members. In this manner, the examples of application of transparent resins to the optical members have been improved intensively.
A transparent resin optical member is manufactured through processes of heating/melting, casting, and cooling/solidification by the use of a machine for normal resin molding such as injection molding or extrusion molding.
Recent resin optical members have been still more reduced in thickness, increased in size, and becoming finer in surface shape. In order to form a calculated optical shape in the resin molding machine, it has become necessary to reduce the viscosity of a molten resin to thereby ensure fluidity.
As a method of reducing the melting viscosity of a resin, there is a method of changing the properties of the resin itself, but a method of easily reducing the viscosity of the resin material is to raise the temperature during melting. However, raising the heating temperature of the resin also represents approaching a decomposition degradation temperature of the resin.
Transparency is important for an optical member. In the resin molding process, a resin that is heated to melt becomes more liable to be decomposed and degraded as the temperature rises. Further, when oxygen exists during high-temperature melting, the molten resin reacts with the oxygen to be easily oxidatively degraded.
Normally, the oxidative degradation temperature of a resin is lower than the decomposition degradation temperature thereof. The oxidatively degraded resin is subjected to coloring and a change in refractive index. Such a resin remains in a molded article as it is and becomes a contaminant to reduce the transparency and cause quality degradation. Further, a resin becomes more susceptible to oxidative degradation as a residence time thereof at high temperature increases.
The oxidatively degraded contaminant remaining in the molded article is called burn, sunspot, yellow, stones, fisheye, gel, or the like and is a major factor for molding failure.
Recently, as a countermeasure therefore, the inside of a resin molding machine is sealed with nitrogen or a resin molding machine with a structure for forcibly exhausting the air containing oxygen has been developed and utilized.
However, it is impossible to completely remove oxygen in the resin molding machine and thus to completely prevent the failure. Further, if the residence time in the high-temperature melting state is shortened, a non-molten resin is extruded into a molding article as it is to cause failure and, therefore, it is quite difficult to find the optimal molding conditions.
The cause of the occurrence of the oxidatively degraded matter such as the burn is associated with the surface roughness of materials of a cylinder, a screw, a nozzle, a die, and so on that contact with the molten resin, among members constituting the resin molding machine. If there are irregularities on the surfaces thereof, the molten resin stays at such portions over the long term, degrades, and peels off some time later to get mixed into a molding article. Therefore, normally, these surfaces are sufficiently polished. If it is still insufficient, plating or coating is applied to flatten the surfaces.
Further, even when the surface is sufficiently flattened, if affinity between the resin and the member surface is strong, adhesion and staying of the resin are liable to occur to cause the oxidative degradation thereof. This is a problem that also arises with a metal mold in an injection molding machine.
The oxidative degradation of the resin adhered to and staying on the member surface is accelerated by the catalytic properties of the surface. Further, it is possible that the plating or coating applied to the surface peels off physically or at times chemically and gets mixed into a molding article, which thus also causes failure. Therefore, it is necessary to sufficiently examine a material to be used for the surface. However, optimal measures that satisfy all have not yet been developed.
It is an object of this invention to provide a resin molding machine using a metal member applied with a treatment for a passive surface that has a low affinity with a resin, that is inactive, and that does not peel off, thereby preventing generation of a contaminant caused by degradation of a molten resin and introduced into a molding article so as to enable remarkable improvement in yield.
Further, it is an object of this invention to provide a member for a resin molding machine using a metal member applied with a treatment for a passive surface that has a low affinity with a resin, that is inactive, and that does not peel off, thereby preventing generation of a contaminant caused by degradation of a molten resin and introduced into a molding article so as to enable remarkable improvement in yield.
Moreover, it is an object of this invention to provide a member for a resin molding machine which is necessary for molding with high yield a resin optical molded article for a large-size flat panel liquid crystal display having a screen diagonal of 28 inches or more.